Process of alkaline fiber liberation



GEORGE .A.. RICHTER, F BERLIN, NEW HAMPSHIRE,

Patented Sept. 27, 1932 UNITED STATES PATENT-OFFICE nssmnon r0 nnown comrm, or

BERLIN, NEW HAMPSHIRE, A GORIORATION OF HAINE PROCESS or ALKALINE FIBER nmnaa'non No Drawing.

This invention relates to the production from raw cellulosic material, such as wood of cellulosic pulps, by a two-step process of alkaline fiber liberation in which the alkaline liquors employed in both ste s to dissolve the cementitions content 0 the raw cellulosic material consist of alkaline solutionssimilar in chemicals used to those employed in the so-called soda and kraft processes.

In the so-called soda and kraft processes of fiber liberation, the raw cellulosic material is cooked in comparatively dilute alkae liquors under conditions of high temperature and pressure until fiber isolation has been accomplished. By neither thesoda process, herein a solution of caustic soda serves as e cooking liquor, nor by the kraft process, wherein a solution of both caustic soda and sodium sulphide serves as the cooking liquor, is it possible to produce pulps of low pentosan content, as dilute alkaline liquors, especially kraft liquors, are comparatively poor solvents for pentosans, even at elevated temperatures.

The object of the present invention is to make possible an economical utilization of the usua fiber liberating alkalies', such as employe in the soda or kr'aft processes, for the production of cellulose pulps having a much lower pentosan content than that of the usual soda or kraft piilps and excelling the usual soda or kraft pulps in other imortant respects, such as in softness, easy leachability, and suitability for conversion into cellulose derivatives. In accordance with the.process of the present invention, raw cellulosic material, such as chipped wood, is steeped for a considerable period of time in an alkaline liquor of high'alka-.

linity but at a temperature little, if any,

above that of the prevailing atmosphere. The free liquor is then separated from the cellulosic material and the cellulosic material is cookedto complete fiber liberation in a comparatively dilute alkaline liquor, which may include residual alkali associated with such material, a suitable amou'ntof water,

and added alkali.

Such atwo-step process has many features of advantage, over the Application filed February-BB, 1930. Serial No. 482,272.

I usual one-step processes of alkali fiber liberation. Thus, I- have found that liquors of high alkalinity extract substantial amounts of lignin, pentosans, less resistant celluloses, and resinous material'from the raw wood at comparatively low temperatures, and, further, that the extraction of pentosans is favored y the low temperature. Hence the subsequent cooking of the wood to complete fiber liberation may be carried out in liquors of much lower alkalinity than that of the usual alkaline cooking liquors, as the partially extracted wood lends itself to much.

easier pulping than raw wood. The iesulting'product is of an alpha cellulose content significantly higher than that of the usual commercial wood pulps, such as sulphite, soda, or kraft, and after bleaching is far more valuable as a raw material for'papermaking and'in-the preparation of cellulose derivatives. An im ortant feature of a process such as described is that it may be carried out in a single alkaline digester, the concentratedliquor employed for the initial treat-= ment being drained from the cellulosic material after'the treatment, and the requisite amount of-water containing alkali, if desired, then being added to the cellulosicmaterial to produce a suitable dilute alkaline liquor in which cooking to complete fiber liberation may be effected. If desired,- however, the initial treatment may be carried out in o n tanks, whereupon after this treatment as what higher temperatures, say, 50 0., particularly when sodium sulphide is present therein, as this chemical is lignin than is caustic soda, especially at temperatures somewhat higher than room temperature. At room temperature or below, a

. maintained at room temperature or at some-.

more active upon liquor containing only caustic soda is especially active upon pentosans, so that when such a liquor is employed, a suitable oxidant may be added thereto to accelerate the removal of lignin 'at such temperature,the permanganate, hypochlorite, or peroxide of sodium beivrgespeclally suitable for this purpose.

herrwood is used as the raw cellulosic material, it is preferable to reduce it to chips of a size much smaller than those prepared for the usual chemical cooking processes. Thus, whereas the usual chip size ranges from -inch to 1-inch in-length, it is preferable to use chips of about fl length in the process of the present invention, as such chipsmake possible the shortening of the time required for the initial treatment and subsequent cooking. The'time of the initial treatment may be from six to twenty-four hours, depending .upon other factors, such' as the strength. and temperature of the liquor, the conditions under which subsequent cooking is to be efi'ected, and the characteristics of the final roduct to be attained. .Under a given set factors, however, the time required for the initial treatment may shortened by using small size chips or b accelerating the impregnation of the chips y confining them and forcing the treating liquor thereinto under the pressure of hydraulic pumps. When the product need not have papermaking characteristics, as, for example, when intended for the preparation of cellulose derivatives, the liquor may contain more than 25% caustic soda equivalent, but liquors of such strength are undesirable from the standpoint that, aside from injuring the fiber for papermaking, they react upon the'cellulose itself and materially lower the yield of product.

When the initial treatment has been completed, the liquor is yellow or light brown in color and the chips have a corresponding color and are slippery to the touch. The liquor is drained from the mass of chips and after suitable fortification with alkali may be reused for the treatment of a fresh batch of chips, as the alkali consumption ina single use is relativel small. 7 The alkali-soaked chips may then e washed, if necessary, until they have a residual alkali content 'suiiicient,

when diluted with the appropriate amount of water, to form a dilute alkaline'liquor such as 1s necessary for cooking to complete fiber liberation. If, however, it is desired to prd duce pulp which may be bleached with ease ina single step to a white product'of high alpha cellulose content, it is preferable to washthe alkall-soaked chips as free as possible from associated alkali, and then to treatthe washed chips with chlorine water, which serves to remove ligneous and coloring matter or to convert this matter into reaction products readily soluble in the dilute alkaline liquor in which subsequent cooking is effected. Because a large proportion of the encrustations of the ment, the wood is considerably more porous than in its raw state and is accordingly penetrated much more rapidly and uniformly by the chlorine water than would be the case were the raw wood treated. The thoroughly washed chips may be steeped at room temperature in chlorine water containing from 4% to 6% chlorine, based on wood, for about four to six hours. Should there be traces of caustic soda in the wood, the chlorine water reacts therewith to produce sodium hypochlorite, the action of which upon ligneous and coloring matter is similar to that of chlorme.

When the washed chips are associated with suilicient alkali to permit carrying out the subsequent cooking operation, cooking is efiected under the desired temperature conditions in a liquor made up of the residuahalkali and water. If, however, the alkali-soaked chips have been thoroughly washed and then treated with chlorine water as hereinbefore described, they are again thoroughly washed after the chlorine water treatment. and before cooking to complete fiber liberation int dilute alkaline liquor. The presence of sodium sulphide in the cooking liquorenhanees the bleachability and strength of thepulp, so that when the initial treatment is carried outwith a solution of caustic soda alone, and no subsequent treatment of the chips with chlorine water is effected, the mass 'of treated chi s is preferably washed to a point where so ium' sulphide may be added along with water thereto to produce a liquor of the desired low alkalinity and at the same time have the desired sodium sulphide content. When, however, the initial treatment is efiected in a solution of both causticsoda and sodium sulhide in which the sodium sulphide is present m such proportion as is vdesired for subsequent cooking, the alkali-soaked chips resulting from the i 'tial treatment may be washed to a point where the addition of water only thereto results in a cooking liquor of the desired alkalinity and sulphidity. In any case, however, cooking to complete fiber liberation is efi'ected in a li uor of much lower alkalinity then that of t e li'quors employed in the production of soda or kraft pulp, for instance, in a liquor of an alkalinity about one-half that. of the usual liquor employed in the production of kraft gulp. A typical proce ure fallingwithin the purried out substantially as follows. Wood chips,'e. g spruce, of about A; to -inch length, are steeped in a caustic soda solution of 18% strength at 20 CI-for twelve hours, at

the end of which time the chips and the solu view of the present invention may becarexcess solution and are washed until they.

contain only about 25% to 35% caustic soda, based on wood. The

chips are then transferred to an alkaline digester, if not already there, and sufiicient water is added to .cover the mass. .The digester is closed and the charge therein is heated to a temperature of about "335 ,F.,-

- charge is maintained for three hours to commating that of a usual, unbleached kraftpulp, but its pentosan content is much lower,amounting toonly about 3% to 4%, as against the?8% to 10% pentosan content of the usual kraft pulp. Such a product may be used as such for papermaking, but when a white product is desired, it may be bleached with a hypochlorite li nor, as usual. When a white product is deslred without decrease in alpha cellulose content, the unbleached pulp is preferablysubjected to chemical rening prior. to the bleaching operation. Such refining may. include the treatment of c the unbleached pulp with chlorine water colitaining, say, 3% to 5% chlorine, based on the weight of 'pulp at room temperature, for about two hours, at the end of which time the chlorine is practically consumed by reaction upon lignin and other coloring matter present in the pulp. The pulp is then washed free of chlorinewater and soluble reaction products, whereupon'it is digested in a dilute solution of a suitable alkali, for example, in a solution containing 5% to'10% caustic soda, based on pulp, at 210 to 212 for about four to five hours. The solution reacts upon and dissolves insoluble chlorinated coloring matter and other non-alpha cellulose components present in the pulp, sothat the resulting product when washed free of alkaline liquor re quires a hypochlorite liquor containing only 3% to 4% bleach, based on pulp, to become converted into a purewhite product. Preferably, the bleach liquor contains, say, 4% to caustic soda, which is su cient to maintain a condition of distinct alkalinit throughout thev bleachin'gio eration, which condition militates against t e formation of oxycelluloses or other undesirable reaction products. The bleached product has analpha cellulose content of about 95%, or even higher, or unbleached pulp resulting from the two step process of fiber liberation. able for use as a cotton fiber substitute, 'both in the manufacture of high grade papers. and

-in the preparation of cellulose derivatives.-

, When the finished pulp is intended priat which temperature thehence comparable to the and is thus more refined than the raw It is suitmarily as a papermaking raw material,-it is preferable that cooking to complete fiber liberation be efi'ected in a liquor containing sodium sulphide, as this chemical enhances the papermaking characteristics, i. e., the strength and tear resistance of the pulp. To this end, the alkali-soaked chips from the initial treatment may be washed until, say, only 10% to 15% caustic soda remains therein, whereupon cooking to complete fiber liberation may e efi'ected in a liquor made up not only of the residual caustic soda associated with the chips, but also with about 10% to 15% sodium sulphide, based on wood. The

raw-pulp resulting from cooking in such a liquor has papermaking characteristics comparable to those of the best grade kraft pulp. It may be used as such, bleached, or refined and then bleached, as already described.

In addition to those advantages hereinbefore enumerated, the-process of the present invention requires the utilization of comparatively little heat in producing a product characterized by its high content of alpha cellulose.

Thus, the initial treatment takes place with the application of little, if any,

- heat, and the subsequent cooking to complete fiber liberation is accomplished in of time notably shorter than that of the usual chemical pulping processes. The process of the present invention may be expediently and economically practised in those mills where soda or kraft pulps are being produced, with little, if any, change in equipment or raw materials, as the regenerated or 'a period .white liquors available in such mills are precisely the liquors required in the process. Moreover, the spent liquors produced in the practice of the present process may be mixed with the spent liquors produced in the soda or kraft mills, and the sodium constituent of the Ifiixed liquors recovered in one recovery apparatus.

I am aware of the fact that it has been proposed to impregnate wood chips or other raw cellulosic material with .the alkali necessary for cooking to complete fiber liberation by soaking in liquors of suitable strength and then cooking the alkali-impregnated wood in water; It has not heretofore been recognized, however, that alkaline liquors of above 10% caustic soda strength are espe I, cially effective in extracting undesirable components, especially pentosans, from wood chips, at about room temperature, and that if the chips are permitted to remain in contact with excess liquor of such strength for a sufiicie-nt period of time, subsequent cooking to complete fiber liberation maybe effected in a liquor of about half the alkali content of the usual alkaline cooking liquors, to produce a pulp characterized by its high alpha cellulose content, its low pentosan content, and its excellence as a raw material both for papermaking and for conversion line liquor of much lower strength to complete fiber liberation.

2. A process which comprises steeping about room temperature until considerable of the pentosan icontent of such wood has been removed and such wood has become im-- pre'gnated with solution, and cooking such impregnated chips in only added water to effect fiber liberation;

.In testimony whereof I have afixed my signature. v

GEORGE A. RICHTER.

chipped wood at not greater than about room temperature in an alkaline liquor or a.

strength of about to caustic soda equivalent until the liquor has extracted sufficient encrusting material from the wood. to have a decided coloration, draining the free liquor from the wood, washing the wood until it contains only about 25% alkali based on wood, and cooking the wood to complete fiber liberation in an alkaline liquor formed by the addition of only water to such wood.

3; A process which comprises steeping chipped wood at not greater than about room temperature in an alkaline liquor of a strength in excess of 10% caustic soda equivalent for over six hours, and then cooking the wood at elevated temperature and under pressure to complete fiber liberation in an alkaline cooking liquor containing only about 25% caustic soda equivalent, based on wood.

4. A process which comprises steeping chipped wood at about room'temperature 1n an alkaline liquor of a strength in excess of 10% caustic soda equivalent for over six hours, and then cooking the wood at elevated temperature and under pressure to complete fiber liberation in an alkaline cooking liquor containing sodium sulphide.

5. A process WhlCh comprises steeping I chipped wood at about room temperature 1n large proportion of the ericrustations of such wood has been dissolved, washing, treating.

with chlorine water toreact upon ligneous and other coloring matter inthe wood, again washing, and cookingin analkaline liquor of low alkalinity to complete fiber liberation.

6. A process which comprises steeping chipped wood in a caustic soda solution of mercerizing strength at not greater than 'about room temperature until considerable of the pentosan content ofv such wood has been removed and such wood has become impregnated with solution, and cooking suchalkaline liquor of impregnated chips in much lower strength il efiect fiber liberap 131011. 7 A process which cpmprises steeping chipped wood in a caustic soda solution of mercerizing strength at not greater than.

,an alkaline liquor of high alkalinity until a v 

